Backup to Clone

My wife is leaving her teaching job. She has a desktop iMac computer and a laptop MacBook supplied by the school. She has to return them both so that they can be fully erased and set up for the teacher who will replacing her. To be sure that we are not missing any important folders or e-mails, I have cloned each computer onto a USB external drive. She will be using a MacBook Air. The USB drive from the iMac plugs into the Mac Air and I can boot from it and run it as if it were the iMac. The drive from the MacBook however will not boot. In Preferences/Startup Disc it does not appear as a choice. I have tried cloning again with Carbon Copy Cloner and SuperDuper but no success.

Rename the users on your teacher's macs, so that say your wife is called alice and she has an alice user account on her own mac (the air)

On the imac rename the user to alice-imac (using the Users and Groups prefs pane)
On the MacBook rename her user account alice-macbook

reclone the school macs again

Then you can use migration assistant on her new Air to bring back those two user accounts without getting a user clash. You can merge any files and folders into her own account at your leisure after giving the school macs back